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  <script src="../../src/array/flatten.js"></script>
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  <script>
    let arr = [1, 2, [3, 4, [5, 6]], 7]
    // 数组扁平化
    console.log(flatten1(arr));//1,2,3,4,5,6,7
    console.log(flatten2(arr));//1,2,3,4,5,6,7
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